Synonyms. Prickly
heat, sudamina, lichen tropicus, strophulus.
Prickly heat is
a common eruption in the tropics, and in temperate climates during the summer,
and occurs as an occupational dermatitis in those exposed unduly to heat and
humidity. Persons who wear woolens and flannels or unduly heavy clothing,
infants and those who are plethoric or obese are predisposed to the disease. It
affects chiefly the bends of the elbows, front and sides of the chest, the
waistline and inframammary regions, but may appear on the backs of the hands,
scalp or other regions except the palms and soles.
Miliaria crystallina is characterized by
small clear superficial vesicles without inflammatory changes. Miliaria rubra
appears as discrete erythematous macules associated with small papules. These
lesions may become confluent. It may also show as distinct clear vesicles such
as occur in miliaria crystallina but each vesicle is surrounded by a halo of
erythema. Pustular miliaria is always preceded by some other skin damage which has produced injury, destruction, or blocking of the sweat
duct or its orifice (pore). Pustular miliaria may occur in association with
contact dermatitis, urticaria, localized neurodermatitis or intertrigo due to
obesity. The pustules develop as a complication during the course of an
inflammatory dermatosis, sometimes several weeks after apparent cure and
discontinuance of all medication.
In the tropics
prickly heat may be accompanied by severe systemic symptoms sometimes leading
to collapse. These are called tropical anhidrosis or sweat retention syndrome,
and are produced by blockage of the sweat gland ducts, thus interfering with
the heat-regulating functions of the sweat glands.
The cause of prickly heat has been studied
intensively since the stimulating discoveries by Sulzberger and his associates
that tropical anhydrotic asthenia and miliaria are caused by a plugging of the
sweat duct orifice by keratinous material. The depth of the obstruction and the
level at which the sweat pours into the periductal tissues are important
factors in determining the clinical types of miliaria. The inflammatory
reaction which could occur as response to the plugging, the pressure, the
sweat, and the constituents in sweat escaping into the periductal tissues or
being resorbed may conceivably produce miliaria rubra without the addition of
infection. However, Sams believes that the effects of persistent high
atmospheric humidity plus high temperatures gradually diminish the oily,
protecting sebaceous film so that perspiration no longer collects at the pores
but is absorbed into the horny layer which becomes macerated and a better
medium for the growth of bacteria and fungi because of hydration. Growth of
these organisms produces inflammation which, together with the swelling caused
by absorbed perspiration, induces closure of the pores. If pore closure is
rapidly effected by acute inflammation reaction many clear vesicles (miliaria
crystallina ) result. If there is a low degree of inflammatory reaction and
sufficient time for growth of bacteria ensues, miliaria rubra develops, which
supports the thesis of O'Brien that in miliaria rubra bacterial infection of
the plugged duct plays a decisive part.
Histopathologically
one finds a dilatation of the sweat ducts, the openings of which are obstructed
by horny plugs. There is also periductal inflammation and signs of damage to
the cells lining the duct.